1. Field of the Invention
This invention relates generally to a filter assembly and, in particular, to a filter for removing contaminants in automatic transmission fluid (ATF) contained in an oil pan.
2. Description of the Prior Art
Hydraulic fluid, which circulates continuously through an automatic transmission and an external oil cooler, reenters a transmission oil pan from the cooler through a filter, whose purpose is to remove contaminants entrained in the fluid. A hydraulic pump is supplied with fluid from the outlet of the filter. A bypass valve may be arranged in parallel flow path with a filter element such that the valve opens and closes in response to differential pressure across the valve.
When the valve is open, however, the filter is bypassed, thereby potentially allowing large contaminants to reenter and recirculate through the transmission.
During cold temperature operation when the valve is open, air may enter the filter assembly and accumulate in large bubbles, which exit the filter and enter the intake of the pump, thereby causing slug-like fluid flow or spikes, which can produce pressure instability downstream of the pump and adversely affect the pump prime.
Undesired high pressure differential across the valve, caused by cold temperature operation and restrictions in the flow path leading to the valve inlet, may cause excessively long periods when the bypass valve is open and the filter is inoperative.
A need exists in the industry for a filter that avoids bypass valve instability due to fluctuations in pump suction flow, excessive differential pressure across a bypass valve, and air entrapment in the housing of a fluid filter.